Remove multi_json dependency

As you are probably aware, [Ruby 1.8 will stop being updated—even for critical security issues—past June](http://www.ruby-lang.org/en/news/2011/10/06/plans-for-1-8-7/). This patch changes the minimum Ruby version to 1.9, as has [already been done in the forthcoming version of Rails](https://github.com/rails/rails/commit/e883c06a4f924cc4ba74efe4dad36394fad26fa0), currently in release candidate.

Since Ruby 1.9 includes `json` in the standard library, `multi_json` is no longer necessary. I am a longtime maintainer of `multi_json` but [will stop supporting the library in June](https://github.com/intridea/multi_json/pull/113#issuecomment-17668823).

This patch replaces `multi_json` with stdlib `json`. It will use the `json` gem if a newer version is available.

I will shortly be submitting similar pull requests to `sprockets`, `uglifier`, and `rails` itself, so `multi_json` need not be a dependency of Rails 4 applications. This should have the effect of making JSON parsing and generation somewhat faster by removing an abstraction layer that is no longer necessary.

One nice side effect of this pull request is that it removes the last runtime dependency of `execjs`.